The Sikorsky S-72 Jet-Assisted X-Wing Helicopter.

Tip-Jet Helicopters
Tech Level: 9
X-Wing Helicopters
Tech Level: 9
Canard Rotor-Wing Helicopters
Tech Level: 12

A jetcopter is an informal term applied to any vehicle that mounts both jet engines and helicopter rotors. The idea is to create a vertical take off and landing vehicle that is also capable of high air speeds. Attempts through the years have met with mixed results. Advanced forms of jetcopters are occasionally seen in science fiction, such as the classic tabletop RPGs Star Frontiers and 2300 AD.

Tech Level: 9
A proposed commercial version pf Swisscopter America's Tip-Jet Dragonfly Helicopter. Image copyright Swisscopter America.

One of the main hurdles in designing a conventional helicopter is the phenomenon of torque. Most helicopter blades are spun by a central shaft revved up by an engine in the main fuselage of the helicopter. The force of the rapidly spinning shaft makes the body of the helicopter want to spin in the opposite direction. Thus, most such craft have a tail rotor angled to the side to counteract this.

But its possible to move the engine from the helicopter body to the blades themselves, and eliminate this problem completely. One well-proven method for doing so is to mount small jet engines on the tip of each rotor blade, with fuel fed to them through lines in the blades.

These tip-jet helicopters were experimented with in the 1950s and 1960s. The design proved workable and practical, and showed impressive potential lifting capacity. However, they were never able to exceed the efficiency and range of conventional helicopters.

They also had a problem with autorotation. If a conventional helicopter’s engine fails, its blades are designed to keep spinning from their motion through the air. This creates enough lift so that a skilled pilot may still land the craft safely. However, with tip-jet helicopters, the jet pods at the rotor tips created too much drag and slowed the unpowered blades down too much for autorotation to be very effective.

The concept however has been revived and upgraded recently with the Swisscopter America Inc.’s new Dragonfly helicopter. The tip jets, actually high-efficiency H2O2 rockets, are more efficient and powerful than the old style jet pods, and their smaller profile means a much less impact on autorotation. A tail rotor is still present, but only to help with maneuverability instead of counteracting torque. Without torque, the flight controls can be simplified, and a tip-jet helicopter proves easier to fly and handle. The design is also very environment-friendly; Swisscopter America’s tip jets’ exhaust is water steam.

The demonstrator model carries enough fuel for 50 minute of flight, twice that if an extra tank is carried as cargo. A more streamlined, two-seat model is being tested, and may serve as the basis for future commercially-available models.

Tech Level: 9

X-Wing helicopters are a straight-forward attempt at combining jet aircraft with helicopters. The project’s prototype, dubbed the S-72, was built by Sikorsky Aircraft with assistance from NASA and DARPA. The project ran from 1983 through 1988.

Functioning as a helicopter, the X-Wing aircraft, with rotor blades literally shaped like the capital letter X, could take off, land, and hover normally. Because its blades needed to be held rigid for jet-assisted flight, they weren’t designed to twist to control lift, the way conventional helicopters do. Instead, compressed air fed from the engine is expelled from the length of its blades to create a ‘virtual’ wing surface that can act in the same was as a twisted rotor blade. This compressed air was controlled by computer, to make sure it flowed from the proper edge of the rotors as needed.

When it transitioned to horizontal jet flight, powered by two auxiliary jet engines mounted on either side of the fuselage, its rotors would lock in place and function as additional lifting surfaces, functioning similarly to X-configured wings. Combined with its more conventional small outrigger wings, the S-72 could have obtained speeds in excess of 200 knots. The goal was to push the design so that an X-Wing’s maximum speed could eventually exceed 300 knots, but the program was cancelled due to budgetary reasons before modifications toward this end could be made.

The aircraft encountered difficulties in transitioning from vertical to horizontal flight, especially with the aerodynamic stresses put on the rotor blades as they spun down to lock into place. Sikorsky also ran into a bit of difficulty with the tail design used, which seemed to produce some longitudinal instability when combined with the airflow from the X-wing blades.

The design may be revived in the future, when more advanced materials technology and computer controls could make the concept more viable.

Tech Level: 12
An artist's conception of a two-passenger Canard Rotor-Wing Helicopter. Original artist unknown.

The Canard Rotor-Wing (CRW) design uses concepts from both Tip-Jet and X-Wing helicopters to produce a comprehensive VTOL aircraft, theoretically capable of both the agility of a helicopter and the speed of a jet aircraft.

The vehicle’s main airfoil/wing is designed to spin like a helicopter’s rotor. Exhaust from the jet engine is directed through nozzles at the rotor’s wingtips in order to make the airfoil spin. Like with tip-jet helicopters, there is no torque effect and the need for a tail rotor is eliminated.

When the aircraft is ready for horizontal flight, the airfoil locks into place to act as conventional wings, and the jet thrust is redirected rearward through a conventional engine nozzle. Ideally, aircraft using this design may exceed airspeeds of 375 knots.

Working together, Boeing and DARPA (Defense Advanced Research Projects Agency), developed the technology under the X-50 program that ran from 1998 through 2006. The program intended to develop Canard Rotor-Wing UAVs first that could be deployed from small naval ships, then to eventually expand the design up to manned versions that could escort larger VTOL craft such as the V-22 Osprey.

Two prototypes were created, but unfortunately both crashed, one in 2004 and the other in 2006. The second crash was determined to be due to aerodynamic complications in the changeover from vertical to horizontal flight, something that has bedeviled many VTOL aircraft concepts. The X-50 program was cancelled after the loss of the second prototype. However, the technology may be revived in the future if the design and flight transition issues can be resolved.


Article added 06/16/10